The present invention relates to a temperature measuring method employing a distributed optical fiber temperature sensor. More particularly, it relates to a distributed optical fiber temperature sensor and a temperature-measuring method, whereby the influence of the attenuation difference in the optical fiber to be measured, which creates a problem when the ratio of the Stokes light and the anti-Stokes light is taken, is corrected so that the temperature distribution in a long distance optical fiber can be measured with low precision.
A block diagram of a conventional distributed optical fiber temperature sensor is shown in FIG. 5. Laser pulses generated from a laser pulser 10 of a light source, are input into an optical fiber 12 to be measured, and Raman back scattered light generated in the optical fiber 12 is returned to the incident end. Such Raman back scattered light is introduced by an optical directional coupler 11 to a measuring apparatus, whereby the Stokes light and the anti-Stokes light in the Raman back scattered light are separated by a filter 13, detected, and converted, respectively, to electrical signals in proportion to their associated amplitudes by the respective photo-electric converters 14 and 14'. Such electrical signals are amplified by the respective preamplifiers 15 and 15' and subjected to a prescribed number of averaging treatment operations by an averager 16. The resulting average-treated signal is transmitted to a signal processing unit 17, whereby the delay time and the ratio of the amplitudes of the Stokes light and the anti-Stokes light are calculated, and the temperature distribution is output.
In the calculation of the ratio of the amplitudes of the Stokes light and the anti-Stokes light, it is difficult to obtain the ratio of the absolute amplitudes. Therefore the ratio of the amplitudes of the strokes and antistrokes light is obtained from the ratio of the relative amplitudes (hereinafter sometimes referred to as "the relative ratio of amplitudes") by the following equation by Dakin (Dakin, J. P. Pratt, D. J., Bibby, G. W., Ross, J. N. "Temperature distribution measurement using Raman ratio thermometry", SPIE Vol 566, Fiber Optic and Laser Sensors III 249 (1985)): ##EQU2## In the above equation, T is the temperature to be measured, .THETA. is the reference temperature, at a reference measuring point R'(T) is the relative ratio of amplitude at the measuring point, R'(.THETA.) is the relative ratio of amplitudes at the reference temperature point, k is the Boltzmann's constant, h is the Planck's constant, c is the velocity of light, and .nu. is the Raman shift.
The incident laser pulses from the light source, such as a semiconductor laser, naturally undergo attenuation during transmission through the optical fiber to be measured. The influence of this attenuation can be avoided by taking the ratio of the Stokes light and the anti-Stokes light. However, the influence of the attenuation difference between the Stokes light and the anti-Stokes light, when the Raman scattering is generated by the incident wave and the scattered light is transmitted backwards, can not be avoided simply by taking the ratio. Therefore, a correction is required which takes the influence of this attenuation difference into consideration. If such a correction is not conducted, even if a uniform temperature distribution is measured, a substantially inclined temperature distribution will be output as shown in FIG. 4. Further, the attenuation difference itself has a temperature dependency, and the influence of the temperature dependency will be substantial in the case of a measurement taken over a long distance. As a result, accurate measurement with high precision has been difficult to obtain.